The invention relates to a device and a method for the alternating operation of ion sources at mass spectrometers equipped with RF multipole ion guides. By making at least one of the multipole ion guides movable perpendicular to the axis, a vacuum-internal exchange of sources can be performed without venting the vacuum system.
For large biological molecules, which decompose when heated, traditional methods of ionization, such as electron impact ionization, cannot be applied. These species require often a milder method of ionization, using which intact molecular ions can be transferred into the gas phase. There are special ionization methods for this, such as electrospray ionization (ESI) or laser desorption ionization (LDI) or also matrix assisted laser desorption ionization (MALDI).
Multiple ionization methods require multiple ion sources for a mass spectrometer. This applies both to ion transmission mass spectrometers, such as a magnet sector or quadrupole mass spectrometers, and ion trap mass spectrometers. In this case, the ion trap can be a Paul quadrupole RF ion trap or an electromagnetic ion cyclotron resonance trap (ICR trap).
Although ions can also be generated in an ion trap, the generation of ions within the measurement cell of the ion trap spectrometers has the disadvantage, that the sample to be ionized has to be introduced into the ion trap. Therefore, the use of all ionization methods directly inside the ion trap is usually more difficult. These methods are frequently applied at xe2x80x9ctrap-externalxe2x80x9d ion sources. Additionally, in case of the Fourier transform ion cyclotron resonance mass spectrometry, the measurements have to be performed in the ultrahigh vacuum conditions such as 10xe2x88x928-10xe2x88x929 mbar, in order to achieve the best results (high resolution, high mass accuracy). The application of the above mentioned ionization methods are, however, associated with a considerable pressure increase in the vacuum system, which is not permitted in the vicinity of the ICR trap and is only tolerated in a trap-external ion source region. Therefore, differentially pumped trap-external ion sources are part of the standard equipment in the high performance FTICR spectrometers. In the following, the trap-external ion sources will just be called xe2x80x9cexternal ion sourcesxe2x80x9d.
In mass spectrometry ion guides have been used for years in order to transfer ions from one part of the mass spectrometer to another part. For transferring the ions formed in an external ion source, various quadrupole ion guid systems have been introduced in the ICR mass spectrometry.
M. W. Senko, C. L. Hendrickson, L. Pasa-Tolic, J. A. Marto, F. M. White, S. Guan and A. G. Marshall describe in their publication in Rapid Communications in Mass Spectrometry 10 1824-1828 (1996) an ion cyclotron resonance mass spectrometer, where the ions, which are generated in a trap-external ion source, are introduced into the ICR trap using an octopole ion guide.
Multipoles connected in series were described in the U.S. Pat. No. 3,473,020 (1969). This patent describes combined multipoles with at least one curved multipole unit.
Shortly after the commercialization of the electrospray ion sources, it is found out that the ions can be introduced into the vacuum system of the mass spectrometer more efficiently using a small multipole unit placed already in the source housing. Therefore, many electrospray ion sources in the market nowadays use a multipole ion guide inside of their housing (U.S. Pat. No. 5,179,278).
In electrospray ionization (ESI) ions are generated at atmospheric pressure using a high voltage (3-6 kV) between an electrospray needle and a counter electrode. In most of the systems immediately after this the ions are sucked through an electrospray capillary into a vacuum. The counter electrode of the electrospray needle is the metallic cap (or a metal coating) at one end of the electrospray capillary. Directly after the exit end of the electrospray capillary one or two skimmers separate the current pressure stage from the next one. The ions are generated in the ESI source at high pressure (atmospheric pressure) but they are transferred to the mass spectrometer at a low pressure (high vacuum). For this, two or sometimes three pumping stages are usually integrated, whereby the pressure at the last stage of the ESI source is reduced down below 10xe2x88x923 mbar. The multipole ion guides in electrospray ion sources are located in this low pressure pumping stage behind the skimmer. The gas stream, which exits the electrospray capillary together with the ions, is xe2x80x9cpeeled offxe2x80x9d by the skimmer, whereby the ions penetrate the hole of the skimmer and fly directly into the multipole ion guide.
An overview article about the mechanism of the electrospray is published by P. Kebarle und L. Tang in xe2x80x9cAnalytical Chemistryxe2x80x9d 65, 972A-986A (1993).
In mass spectrometry laboratories, which work with ICR traps or Paul traps, but also with triple stage quadrupole mass spectrometers, electrospray sources are preferred. The reasons are not only the simple and versatile possibilities of use of an electrospray source including the direct coupling possibility to a liquid chromatograph. From biologically interesting large molecules, such as proteins, electrospray ion sources often generate ions with multiple positive charge or multiple negative charge. The positive ions are usually generated by multiple protonation and the negative ones by loss of protons correspondingly. Consequently, their mass-to-charge ratio (m/z) shifts to much lower mass areas of the mass spectrum, which practically means an extension of the mass range. The mass signals of a 66 times protonated bovine serum albumin (≈66 kDa) appears for example already by m/z≈1000.
On the other hand, in the case of MALDI, multiply charged ions are limited to exceptional cases. Although the MALDI method leads with very low amounts of substance to very good results, it is much more often used with time of flight mass spectrometryxe2x80x94due to its wide mass range - than with ion cyclotron resonance traps or with Paul traps. A MALDI overview article by E. J. Zaluzec, D. A. Gage, J. T. Watson in Protein Expression and Purification 6, 109-123 (1995) reports about the applications of this method for characterization of proteins and peptides.
However, MALDI is also increasingly being used with FTICR mass spectrometers, since these instruments produce results with a mass accuracy unachievable by others. MALDI is also used with RF ion traps.
Ions can be trapped in multipole ion guides, as described in the U.S. Pat. No. 5,179,278 for a multipole ion introduction system however in the case of a linear multipole. On the other hand, the patent DE 196 29 134 describes such a possibility with curved multipole ion guides. For this, apertured end plates are placed at both ends of the ion guide. The ions are reflected back to the middle of the hexapole, if these plates have same sign of charge as the ions to be stored. This way, positive ions are kept in the multipole by using a positive trap voltage. By pulsing the positive voltage down to zero or to small negative values, accumulated ions can be extracted in the corresponding direction.
In the case of mass spectrometers with multiple ion sources appears the problem of changing the source. Nowadays, especially FTICR mass spectrometers are used very often with multipole ion sources. If an ion source of such a versatile mass spectrometer has to be swapped against another one, this is associated with venting of at least a part of the vacuum system of the mass spectrometer. This again costs a certain interruption time.
In the bio-sciences the electrospray source is used primarily. Therefore, mass spectrometers often have an electrospray source, which is constantly in use or on standby. This vacuum-external source is then replacedxe2x80x94as requiredxe2x80x94by another, for instance a MALDI source or an electron impact source. However, in order to install these vacuum-external sources, the vacuum is interrupted, the vacuum-external ion source (ESI) is removed and the new source is mounted.
A proposal to solve this problem is to carefully arrange the placement of the ion sources and equip the system with moveable curved or angled multipole ion guides (German Patent DE 196 29 134). This proposal describes the possibility of connecting fix-mounted ion sources in parallel, of which only one will be in operation at a time. However, the disadvantage here is the placement of the ion sources, which has to be at precise angles and the corresponding adjustment
The objective of the invention is to find a device for rapidly exchanging multiple external ion sources without interrupting the vacuum in the mass spectrometer, and a method for its operation.
The basic idea of the invention is to install two, three or more multipole ion guides in series (multipole sequence) as an ion guide system between a fix-installed ion source and the mass spectrometric analyzer, and make at least one of the multipoles slidable out of its axis, so that another ion source can be inserted in place of this multipole removed by sliding and put into operation.
One of the sources of the mass spectrometer (for example an electrospray source) is located at one end of the multipoles that are placed in series. Consequently, the ions produced in this ion source pass through the entire sequence of the multipole ion guides and transferred this way to the analyzer region of the mass spectrometer. At least one of the movable multipole ion guides can be, however, removed out of the multipole sequence (therefore, out of the axis of this ion transfer system) by sliding off. Additional vacuum-internal but trap-external ion sources, which are movable and are located already in the vacuum system can be slid into the resulting gap to the axis of the ion transfer system and can be put into operation. Ions that are formed in one of these other ion sources will pass now, of course, only through the remaining section of the ion transfer system on their way to the mass spectrometric analyzer.
Using this invention, an ion source exchange in a mass spectrometer can be performed manually or motorized, without having to vent the vacuum system.